Plating films of zinc-nickel alloy are well known as being more corrosion-resistant than zinc plating films and have been increasingly used in recent years, for example, to improve the corrosion resistance of automotive parts and the like.
Methods heretofore proposed for forming a film of zinc-nickel alloy include, for example, electroplating methods using an acid plating bath comprising zinc chloride and nickel chloride (Japanese Examined Patent Publication No. 12343/1985). However, the proposed method has drawbacks. If the method gives a film of zinc-nickel alloy having a thickness of about 5 .mu.m required for prevention of corrosion, the film exhibits reduced flexibility, posing the following problems. For example, if an automotive part with the 5 .mu.m-thick film formed thereon has been installed in an automotive body, the stress applied during installation causes cracking in the film. In this case, the zinc-nickel alloy film is less corrosion-resistant than a zinc film because of this defect as well as due to its lesser degree of sacrificial anticorrosive action on an iron substrate than the zinc film. On the other hand, if the zinc-nickel alloy film has a thickness of less than 5 .mu.m, no cracking would occur during installation but the film is not fully satisfactory in corrosion resistance. Further a film of locally irregular thickness is formed by the method because electroplating unavoidably entails an uneven current density at the surface of substrate to be electroplated. For example, the film is imparted an unnecessarily large thickness over a substrate portion of higher current density where cracking is more likely to develop in installation. More disadvantageously said acid plating bath contains a large amount of chloride which tends to cause corrosion in the plating equipment due to their marked corrosive property.